In xerographic print engines, a tone reproduction curve (TRC) is important in controlling the image quality of the output. An image input to be copied or printed has a specific tone reproduction curve. The image output terminal outputting a desired image has an intrinsic tone reproduction curve. If the image output terminal is allowed to operate uncontrolled, the tone reproduction curve of the image output by the image output terminal will distort the rendition of the image. Thus, an image output terminal should be controlled to match its intrinsic tone reproduction curve to the tone reproduction curve of the image input. An intrinsic tone reproduction curve of an image output terminal may vary due to changes in such uncontrollable variables such as humidity or temperature and the age of the xerographic materials, i.e., the numbers of prints made since the developer, the photoreceptor, etc. were new.
Solid developed mass per unit area (DMA) control is a critical part of TRC control. If the DMA is too low then the images will be too light and customers will be dissatisfied. On the other hand, if the DMA is too high, then other xerographic or image quality problems, such as poor transfer efficiency, fusing defects, or toner scatter on lines, etc., can occur. High DMA will also increase the total cost to owner. Maintaining a constant DMA or a low variation of DMA has always been a challenge in xerographic process controls design.
In addition, in copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to artificially create a “test patch” of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured by a suitable sensor to determine the effectiveness of the printing process in placing this printing material on the print sheet. In such a case, the optical device for determining the density of toner on the test patch, which is often referred to as a “densitometer,” is disposed along the path of the photoreceptor, directly downstream of the development unit. For example, see U.S. Pat. No. 5,162,874, herein incorporated by reference.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent. Test patches are used to measure the deposition of toner on paper to measure and control the tone reproduction curve.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch. The sensor readings are then used to make suitable adjustments to the system such as changing developer bias to maintain consistent quality.
Typically each patch is about an inch square that is printed as a uniform solid half tone or background area. This practice enables the sensor to read one value on the tone reproduction curve for each test patch.
The Xerox iGen3® digital printing press includes a densitometer, for example, an Enhanced Tone Area Coverage (ETAC) sensor, as disclosed in U.S. Pat. No. 6,462,821, and herein incorporated by reference. As shown in FIG. 1A, the ETAC sensor contains an illuminator, e.g., a single light emitting diode (LED) 2, and two sensors, a diffuse sensor 3diff and a specular sensor 3spec. When the ETAC is located at the optimal distance d from the photoreceptor 1 the LED 2 is at a 45° angle with respect to diffuse sensor 3diff and at a 90° angle with respect to specular sensor 3spec.
A processor (not shown) is provided to both calibrate the sensors and to process the reflectance data detected by the sensors. It may be dedicated hardware like ASICs or FPGAs, software, or a combination of dedicated hardware and software. For the different applications the basic algorithm for extracting the specular and diffuse components would be the same but the analysis for the particular applications may vary.
While specular light is reflected only at 90°, diffuse light is reflected over a wide range of angles, including the specular angle. The specular reflection, which is sensitive to the area covered by the toner is used to control the Tone Reproduction Curve (TRC), and hence the colors printed by the printing press. Unfortunately, some of the diffuse light reflected from the toner will be reflected at the specular angle. The amount of diffuse reflection depends on manufacturing parameters and on the particular spacing between the sensor and photoreceptor. While varying the ETAC spacing is not a desirable feature, it is nonetheless an unavoidable outcome of manufacturing tolerances. This variation is a contributor to machine-to-machine color variation in the field.
During operation of the printing press, the toner will absorb and scatter a portion of the light from LED 2, such that some of the light is not reflected at the specular angle. Black toner absorbs more light at the LED 2 wavelength, and scatters minimally. On the other hand, however, colored toner does not absorb all of the light, and scatters a substantial amount of it, so that it is widely spread over a range of angles.
The densitometer may be calibrated by determining an uncompensated specular sensor value, i.e., the specular light component of the total light collected from a central (specular) sensor. When the ETAC sensor is manufactured and/or subsequently calibrated, the light detected by diffuse sensor is internally subtracted from the specular sensor signal. Moreover, in order to compensate for environmental conditions and differences between individual machines, only a fraction of the diffuse signal may be internally subtracted, corresponding to a compensation ratio of the voltages of the specular and sensor signals.
Since the amount of diffuse light reflected at the specular angle is generally small, the residual error in the specular sensor signal, i.e., the amount of diffuse light actually incident on the specular sensor 3spec, is usually assumed to be negligible. For example, FIG. 2 depicts a plot of Vspec and Vdiff, and the sum of Vspec and Vdiff. Since the value of Vspec plus Vdiff is substantially the same as Vspec, the residual error in the specular sensor signal has generally been ignored.
In operation of the printing press, the area covered by toner is determined by dividing the amount of light absorbed by the toner from the total amount of light reflected from the photoreceptor. This is referred to as the Fractional Area Coverage (FAC). The measured Fractional Area Coverage (mFAC) is calculated based on the specular voltage, according to Equation 1:mFAC=(Vcb−Vspec)/(Vcb−V01x)  (1)                where: Vcb is the voltage returned from the specular sensor 3spec from a clean photoreceptor (i.e., one having no toner on it);                    V01x is the background noise signal returned from the specular sensor 3spec with the LED 2 turned off. For example, the specular sensor 3spec generally returns a signal of approximately +0.5 V in the absence of any light; and            Vspec is the specular voltage returned from the patch being measured less the value internally subtracted by the ETAC sensor.                        
Unfortunately, the impact of a diffuse balance error is magnified due to variance in the spacing of the ETAC sensor from the photoreceptor 1. As shown in FIG. 1B, as the distance d′ between the sensors 3spec, 3diff and the photoreceptor 1 varies due to manufacturing tolerances, the LED 2 is no longer at a 45° angle with respect to diffuse sensor 3diff and at a 90° angle with respect to specular sensor 3spec. This may increase the angle of the specular sensor 3spec such that it becomes closer to the diffuse angle, and more diffuse light is gathered by the specular sensor 3spec. FIG. 3 shows a plot of the angles of the specular and diffuse sensors with respect to the spacing of the ETAC sensor. In addition, as the specular angle moves off a right-angle (90°) from the LED 2 intensity must be increased to give the same specular signal, which also increases the total diffuse light output.
FIG. 4 shows a plot of a DMA sweep and how these problems are manifested. For example, if the ETAC sensors 3spec, 3diff are too close to the photoreceptor 1, the amount of diffuse light subtracted internally may be greater than the actual amount of diffuse light at the specular sensor 3spec. This causes a “blind balance” and at high DMA the ETAC sensor will rail, i.e., hit a maximum, at a value of 1. Conversely, if the ETAC sensor is further away from the photoreceptor 1, too little diffuse light is subtracted, and the FAC hits a maximum near 0.6 DMA then curves downward. The error in the measured FAC is most evident at high DMA.
In order to correct for this error in the measured FAC, Xerox Corporation uses a software algorithm, which divides the measured FAC, mFAC by the maximum FAC value measured during a DMA sweep, SpecFACmax, according to Equation 2:SpecFACmax corrected FAC=mFAC/SpecFACmax  (2)
FIG. 5 shows that this correction method is effective in resealing FAC values between 0 and 1, which is important for solid area DMA control. However, the diffuse channel is calibrated using the specular data, and this calibration is extremely sensitive to variations in FAC near 1. Initial estimates of the improvement in DMA accuracy expected from resealing by SpecFACmax assumed that this resealing would eliminate errors due to ETAC spacing variation. FIG. 6 shows, however, that for actual data resealing alone does not eliminate the error in DMA accuracy. Scaling decreases the maximum error, and brings the average error close to zero; but the error introduced in the mid and low patches can be greater than the original uncorrected error.
FIG. 7 shows that varying the ETAC spacing and SpecFACmax, and then measuring vacuum DMA, decreases the error, but does not eliminate DMA variation. Furthermore, FIG. 8 shows that for test data for CMYK color printing the amount of DMA variation after SpecFACmax correction is still about half the uncorrected variation.